The present invention resides in a combined cycle power plant, particularly an integrated steam and combustion turbine power plant with a highly efficient Brayton/Rankine cycle arrangement.
The efficiency of conventional steam power plants has been improved by integration of a combustion turbine into conventional steam turbine plants with gas, oil or coal fired boilers which include combustion air preheaters in their exhaust duct to recover as much as possible heat from the boiler exhaust by transferring it to the combustion air for the boiler. However, in a combined cycle plant, combustion air preheaters are not needed since the boilers receive the hot exhaust gases of the associated gas turbines as combustion gases for the boilers.
In order to recover the energy in the boiler exhaust in such combined arrangements, generally, a portion or all of the feedwater of the integrated steam cycle is passed through a stack gas cooler which is installed downstream of the boiler. Often a boiler by-pass is provided by way of which the exhaust gases from the gas turbine can by-pass the boiler when the load on the boiler is relatively low. However, all exhaust gas passes through the stack gas cooler which is located downstream of the juncture where any exhaust gas by-passing the boiler is recombined with the boiler discharge gas. The stack gas cooler is therefore always exposed to the full gas turbine exhaust flow unless the stack gas cooler includes a controllable by-pass flow structure.
It is to be noted that single, large industrial combustion turbines have only a limited capability of adjusting their volumetric flow rate. Thus, the energy available from the stack gas cooler does not vary much over the plant load range. However, the feedwater flow through the stack gas cooler decreases substantially as plant load is reduced. Therefore, even with a slight reduction in plant load the temperature of the feedwater in the stack gas cooler may reach its saturation point so that steaming may occur as the feedwater flows from the stack gas cooler to the boiler. To avoid this undesirable situation, full load feedwater flow through the stack gas cooler is generally maintained at all times and, during part load, when the boiler requires only part of the feed water, the balance is dumped into the condenser. The energy of the feedwater dumped into the condenser is not recovered but is lost which results in reduced operating efficiencies.
It is therefore the principal object of the present invention to provide a combined steam and combustion turbine power plant in which the efficiency does not suffer during part load operation as a result of combustion turbine excess exhaust heat generation.